Problem: Solve for $x$ : $x^2 - 18x + 81 = 0$
Answer: The coefficient on the $x$ term is $-18$ and the constant term is $81$ , so we need to find two numbers that add up to $-18$ and multiply to $81$ The number $-9$ used twice satisfies both conditions: $ {-9} + {-9} = {-18} $ $ {-9} \times {-9} = {81} $ So $(x {-9})^2 = 0$ $x - 9 = 0$ Thus, $x = 9$ is the solution.